Thermostats have been known and used extensively to control the circulation of coolant in internal combustion engines. In the past, the thermostats have taken the form of valves which are immersed in the coolant in, for example, a coolant conduit. Most commonly the valves include a valve member which spans the conduit and sits against a valve seat. Thus, in the closed position the valve substantially blocks the flow of coolant, for example, to the radiator, forcing the coolant to re-circulate within the engine to heat up more quickly.
Typically such valves include a closed body containing a thermally expandable material such as wax, where the closed body is immersed within the coolant fluid. As the fluid temperature rises the wax expands, thrusting out a piston. The piston lifts the valve off the valve seat to allow the coolant to circulate down a new path, such as past a heat exchanger or radiator. This lowers the temperature of the coolant and removes heat from the engine. A spring is provided to urge the valve to a closed position so that in the resting or cooled state the valve is normally closed. Thus, when an engine is first started, the valve will be closed allowing the engine to attain its optimum running temperature more quickly by preventing the circulation of the radiator fluid outside of the engine.
Thermostats, to date, have been designed to permit the engine to operate over time at a constant optimum temperature. The thermostat accomplishes this by opening a valve in the cooling system when the engine temperature, and thus the liquid coolant temperature, rises. Opening the valve permits more flow to a heat exchanger such as a radiator, permitting more heat to be dissipated, which in turn can lower the engine temperature. As the engine temperature drops, and thus the coolant temperature drops, the valve closes, reducing the amount of heat dissipated and again maintaining an optimum operating temperature.
Such prior art thermostats are effective, simple and reliable, but suffer from several drawbacks. One is that the thermostat essentially requires the engine designer to set one optimum engine temperature. However, in practice, the engine operating temperature is known to affect engine performance. Specifically, a hotter running engine produces less in the way of emissions, by permitting more complete combustion which in turn improves fuel economy. A hotter running engine will deliver less power, while a cooler running engine delivers more power. Thus, any single optimum engine temperature is a compromise between power and emissions.
Another drawback is that thermostats are slow to respond. The coolant temperature change is fairly gradual and since the change in coolant temperature controls movement of the piston, the valve only opens slowly. Essentially the response of the thermostat lags the engine demand and thus acts as a dampened system. For example, it might take the thermostat 12 minutes to respond in winter when the engine start is very cold, and about 5 minutes in summer where the engine start temperature is warmer. Sharp changes in engine temperature which arise and then recede quickly are not well managed by the thermostat. However, such sharp changes may occur, for example during acceleration from a stop, when accelerating to pass, or when climbing a hill. Therefore there has been an effort to develop a thermostat which responds, on demand, rather than simply passively following coolant temperature. Of course, such a more responsive thermostat still needs to reliably respond to coolant temperature changes in a manner which prevents overheating.
Various levers and actuators have been proposed to open and close valve elements on demand, but these suffer from various disadvantages. Firstly, they are relatively expensive. Secondly, they involve complex mechanical moving parts which interface with a complex engine control system either of which can fail over time. A failed actuator system could lead to the valve remaining in one position, such as being closed, which in turn could lead to overheating and failure of the engine, which is unacceptable. Thus, electromechanical systems have certain drawbacks.
The present inventors have proposed a thermo-mechanical solution in two prior U.S. patents, namely, U.S. Pat. Nos. 6,598,565 and 6,595,165. These prior patents teach inventions based on a combination of a thermostat valve for blocking and unblocking the flow of fluid, with a thermally activated actuator operatively connected to the valve. The actuator is positioned so as to be partly within the coolant so as to maintain the thermal activation part of the actuator at about the coolant temperature, when not being activated. A separate heater with an electronic heater control is connected to the actuator to cause the temperature of the actuator to be controlled to effect controlled displacement of the piston and thus to permit opening and closing of the valve on demand.
However, while providing an adequate response time in certain circumstances, the performance of the thermo-mechanical actuator is somewhat hindered by the time lag between the need to open the valve and the time needed to change the temperature of the thermally expansive material via the heater. In addition, the teachings require the use of two different actuation temperatures for the valve and for the thermal actuator, which increases the manufacturing complexity and costs. Additionally, the action of the thermo-mechanical actuator is bimodal, in that it is either open or closed—establishing a valve position corresponding to a partially open setting for the fluid flow is difficult to do. What is desirable therefore is a device which is capable of opening and closing the valve on demand, is relatively immediately responsive to short term engine demands, can be used to control the position of the valve to establish a partially open valve position, is reliable and easy to manufacture and avoids the drawbacks of the prior art systems. Most preferably such a device would be safe and would not be likely to fail in a manner that would cause engine overheating, in other words would include a fail safe configuration.